Glacier growth

As the glacier flows downhill, it descends to warmer zones where the snow melts from year to year. The boundary where loss from melting and evaporation equals accumulation from snowfall is called the annual snowline or firn limit-"firn" being the term for partially compacted snow carried over from previous seasons.
The firn limit fluctuates from year to year in response to changes in precipitation and temperature. The firn limit can be as much as 1,000 ft lower in elevation on the shaded north sides of mountain peaks tan on their sunny south sides. For this reason, many present-day glaciers are found on the north-facing mountain slopes.
The section of the glacier through which the maximum amount of ice flows coincides with the firn limit, because as the glacier flows toward the firn limit, it is continually augmented by new net snowfall; and downvalley from the firn limit, more ice is lost by melting and evaporation-together called ablation-each year than is added by snowfall. As the glacier flows downvalley from the firn limit, more and more of the ice ablates, and the glacier grows thinner or narrower, or both. Ultimately a point is reached where the ice front can advance no farther because the ice melts there as rapidly as it is provided by inflow from up glacier. If the yearly rates of accumulation and ablation were constant, this point would be fixed. However, they vary, and for that reason alone the terminus of the glacier is not likely to be fixed in position. As the climate turns warmer or drier, a glacier will gradually waste away, rather than melting catastrophically.